1. Field of the Invention
This invention relates generally to indirectly mounting pressure sensitive Silicon chips to Stainless Steel housings and particularly to mounting such a Silicon chip to Stainless Steel through a holder assembly comprising a Borosilicate glass holder soldered to a Nickel-Iron holder.
2. Description of the Prior Art
In electronic pressure transmitters using pressure sensitive Silicon chips an existing problem is the mounting of the Silicon chip to the corrosion resistant metal housing such as a Stainless Steel housing. The Silicon chip and the Stainless Steel housing of the transmitter have different coefficients of thermal expansion which causes thermal fatigue and cracking of the pressure sensing Silicon chip when directly bonded to the Stainless Steel housing and operated in the operating temperature range of the transmitter (-40.degree. F to +250.degree. F).
To overcome this problem, prior designs utilized a flexible epoxy adhesive to bond the Silicon chip either directly to the Stainless Steel housing or to bond the Silicon chip to a glass tubular holder which was then bonded to the Stainless Steel housing. The flexible epoxy was supposed to absorb any strain from the difference in temperature expansion. The glass holder was found to be, nevertheless, necessary to provide a strong mounting holder for the Silicon chip which had a coefficient of expansion similar to the Silicon chip and which would absorb most of the shock of the expansion differential between the Stainless Steel housing and the Silicon chip without transmitting these forces to the more fragile Silicon chip. However, this type of flexible epoxy bonding was still found to have certain drawbacks. The epoxy seal was found to deteriorate with the passage of time as well as being unable to withstand the temperature cycling of the instrument between the extremes of the operating temperatures of -40.degree. F to 250.degree. F. Other problems with the epoxy seal were found to be poor shock withstanding ability of the seal and a high water absorption by the epoxy which caused the seal to deteriorate and the instrument to either malfunction or become inoperative.
Having found epoxy bonding to be inadequate, brazing of a Silicon chip assembly directly to the Stainless Steel case was considered. However the temperatures required to properly braze the assembly to the Stainless Steel case were found to exceed the maximum temperatures of either the Silicon chip or the Silicon compatible glass chip holder.